查看“统计分析：使用 R/R/矩阵入门”的源代码

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|Much statistical theory uses [[w:matrix algebra|matrix algebra]]. While this book does not require a detailed understanding of matrices, it is useful to know a little about how R handles them.
 
Essentially, a '''matrix''' (plural: matrices) is the two dimensional equivalent of a vector. In other words, it is a rectangular grid of numbers, arranged in rows and columns. In R, a matrix object can be created by the <code>matrix()</code> function, which takes, as a first argument, a vector of numbers with which the matrix is filled, and as the second and third arguments, the number of rows and the number of columns respectively.

R can also use '''array''' objects, which are like matrices, but can have more than 2 dimensions. These are particularly useful for '''tables''': a type of array containing counts of data classified according to various criteria. Examples of these "contingency tables" are the <code>HairEyeColor</code> and <code>Titanic</code> tables shown below.

As with vectors, the indexing operator <code>[]</code> can be used to access individual elements or sets of elements in a matrix or array. This is done by separating the numbers inside the brackets by commas. For example, for matrices, you need to specify the row index then a comma, then the column index. If the row index is blank, it is assumed that you want all the rows, and similarly for the columns.|m <- matrix(1:12, 3, 4)      #Create a 3x4 matrix filled with numbers 1 to 12
m                            #Display it!
m*2                          #Arithmetic, just like with vectors
m[2,3]                       #Pick out a single element (2nd row, 3rd column)
m[1:2, 2:4]                  #Or a range (rows 1 and 2, columns 2, 3, and 4.)
m[,1]                        #If the row index is missing, assume all rows
m[1,]                        #Same for columns
m[,2] <- 99                  #You can assign values to one or more elements
m                            #See!
###Some real data, stored as "arrays"
HairEyeColor                 #A 3D array
HairEyeColor[,,1]            #Select only the males to make it a 2D matrix
Titanic                      #A 4D array
Titanic[1:3,"Male","Adult",] #A matrix of only the adult male passengers
|<pre>> m <- matrix(1:12, 3, 4)      #Create a 3x4 matrix filled with numbers 1 to 12
> m                            #Display it!
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12
> m*2                          #Arithmetic, just like with vectors
     [,1] [,2] [,3] [,4]
[1,]    2    8   14   20
[2,]    4   10   16   22
[3,]    6   12   18   24
> m[2,3]                       #Pick out a single element (2nd row, 3rd column)
[1] 8
> m[1:2, 2:4]                  #Or a range (rows 1 and 2, columns 2, 3, and 4.)
     [,1] [,2] [,3]
[1,]    4    7   10
[2,]    5    8   11
> m[,1]                        #If the row index is missing, assume all rows
[1] 1 2 3
> m[1,]                        #Same for columns
[1]  1  4  7 10
> m[,2] <- 99                  #You can assign values to one or more elements
> m                            #See!
     [,1] [,2] [,3] [,4]
[1,]    1   99    7   10
[2,]    2   99    8   11
[3,]    3   99    9   12
> ###Some real data, stored as "arrays"
> HairEyeColor                 #A 3D array
, , Sex = Male

       Eye
Hair    Brown Blue Hazel Green
  Black    32   11    10     3
  Brown    53   50    25    15
  Red      10   10     7     7
  Blond     3   30     5     8

, , Sex = Female

       Eye
Hair    Brown Blue Hazel Green
  Black    36    9     5     2
  Brown    66   34    29    14
  Red      16    7     7     7
  Blond     4   64     5     8

> HairEyeColor[,,1]            #Select only the males to make it a 2D matrix
       Eye
Hair    Brown Blue Hazel Green
  Black    32   11    10     3
  Brown    53   50    25    15
  Red      10   10     7     7
  Blond     3   30     5     8
> Titanic                      #A 4D array
, , Age = Child, Survived = No

      Sex
Class  Male Female
  1st     0      0
  2nd     0      0
  3rd    35     17
  Crew    0      0

, , Age = Adult, Survived = No

      Sex
Class  Male Female
  1st   118      4
  2nd   154     13
  3rd   387     89
  Crew  670      3

, , Age = Child, Survived = Yes

      Sex
Class  Male Female
  1st     5      1
  2nd    11     13
  3rd    13     14
  Crew    0      0

, , Age = Adult, Survived = Yes

      Sex
Class  Male Female
  1st    57    140
  2nd    14     80
  3rd    75     76
  Crew  192     20

> Titanic[1:3,"Male","Adult",] #A matrix of only the adult male passengers
     Survived
Class  No Yes
  1st 118  57
  2nd 154  14
  3rd 387  75</pre>
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==Notes==
{{Reflist}}
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